Kinetic resolution of axially chiral 2,2′-dihydroxy-1,1′- biaryls by palladium-catalyzed alcoholysis

Article abstract of DOI:10.1021/ja051750h

Palladium-diamine complexes catalyzed kinetic resolution of axially chiral 2,2′-dihydroxy-1,1′-biaryls by alcoholysis of vinyl ethers. The reaction proceeded with high selectivity for various kinds of biaryls. This process is applicable to not only binaph

Full text of DOI:10.1021/ja051750h

Published on Web 07/08/2005
Kinetic Resolution of Axially Chiral 2,2′-Dihydroxy-1,1′-biaryls by
Palladium-Catalyzed Alcoholysis
Hiroshi Aoyama, Makoto Tokunaga,* Junya Kiyosu, Tetsuo Iwasawa, Yasushi Obora, and
Yasushi Tsuji*
Catalysis Research Center and DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity,
SORST and CREST, Japan Science and Technology Corporation (JST), Sapporo 001-0021, Japan
Received March 19, 2005; E-mail: tokunaga@cat.hokudai.ac.jp
Axially chiral biaryls are important modules in synthetic,1a
pharmaceutical,1b material,1c and supramolecular1d chemistry. Es-
Table 1. Kinetic Resolution of rac-1a Catalyzed by Pd(OAc)2
Complexes with Various Kinds of (R,R)-Diaminea
pecially, optically active 2,2′-dihydroxy-1,1′-biaryls have received
primary attention because they have been utilized not only as ligands
but also as precursors for phosphine ligands in catalytic asymmetric
reactions.1a The success of complexes of 2,2′-disubstituted 1,1′-
binaphthyls, in particular, BINOL and BINAP, in giving high
enantioselectivities in numerous catalytic reactions has encouraged
t
conv
(%)b
ee of 1a
(%)c
ee of 2a
(%)c
the synthesis of several related ligands.
d
entry
ligand
(h)
krel
The optically active 2,2′-dihydroxy-1,1′-biaryls have been pro-
vided by conventional optical resolution or chiral pool methods
using stoichiometric chiral sources2 because they cannot be
constructed by the typical catalytic asymmetric reactions, including
hydrogenation and aldol reaction. The enantioselective oxidative
coupling of 2-naphthols is the only way to access them so far.
However, this method is effective only for the building of optically
active BINOL and its analogues.3 The efficient synthesis of optically
active 1,1′-bi-2-phenols has not been attained due to the low
reactivity of phenols. Thus, the development of a novel methodol-
ogy for these classes of compounds is an interesting target in
asymmetric catalysis.
1
2
3
4
5
6
7
8
9
(S)-BINAP
(-)-sparteine
3
4a
4b
4c
4d
4e
4f
137
187
18
22
9
3
2
15
20
64
81
81
68
3
88
79
84
69
1.3
1.4
7.2
16.5
16.0
5.8
43
43
40
19
41
11
41
35
58
50
61
54
16
2
11
56
45
96
62
109
107
46
111
42
1.1
14.7
13.8
18.3
20.3
10
11
4g
4h
61
61
a The reaction was carried out with 1 M solution of 1a, 5 mol % of
Pd(OAc)2, 10 mol % of ligand, and 10 equiv of MeOH in CH2Cl2 at 20
°C. b Calculated from isolated yields of 1a and 2a. c Determined by HPLC.
d From ref 10.
para-position exhibited higher krel value (krel ) 16.0, entry 5) than
meta- (4c, krel ) 5.8, entry 6) and ortho- (4d, krel ) 1.1, entry 7)
positions. The steric and electronic effect of the para-substituent
was investigated (H, Me, tBu, F, and Ph, entries 4, 5, 8, 9, and 10).
The ligand 4g bearing a phenyl group gave slightly higher selectivity
(krel ) 18.3, entry 10) than did 4a, although other ligands showed
lower selectivity. On the basis of this result, we introduced a 2,3,4,5-
tetraphenylphenyl group,7 which exhibited unique steric effect in
a Pd/pyridines-catalyzed aerobic oxidation of alcohols.8 Interest-
We recently reported a catalytic hydrolysis of alkenyl ethers and
esters and their application to hydrolytic kinetic resolution4 of some
racemic vinyl ethers.5 Although various metal complexes, such as
PdII, PtII, HgII, CuII, CoIII, RuII, and ScIII, showed activity in the
hydrolysis of vinyl ethers, only (salen)Co complexes catalyzed the
asymmetric hydrolysis, giving moderate selectivity (krel ) 10).
Herein, we report a Pd-catalyzed kinetic resolution of vinyl ethers
of axially chiral 2,2′-dihydroxy-1,1′-biaryls with high selectivity
and generality. This is the first example for the efficient preparation
of optically active 1,1′-bi-2-phenols by a catalytic system.
Our attempts started with the hydrolysis of BINOL vinyl ethers
using (salen)Co and chiral ligands-Pd systems. The optimization
study revealed that chiral secondary diamines 4/Pd-catalyzed
methanolysis6 rather than hydrolysis gave favorable results. The
kinetic resolution of racemic 1a by Pd(OAc)2 complexes was
performed under the methanolysis condition (Table 1). A diamine
ligand 4a derived from (R,R)-1,2-cyclohexanediamine showed
higher selectivity (krel ) 16.5, entry 4) than that of another diamine
ligand 3 (krel ) 7.2, entry 3) and typical chiral ligands, such as
BINAP (krel ) 1.3, entry 1) and sparteine (krel ) 1.4, entry 2). The
selectivity was largely influenced by the position of substituents
on the phenyl ring of 4, the ligand 4b having a methyl group at the
ingly, 4h afforded the best result with regard to selectivity (krel
)
20.3) and reactivity (entry 11), while the typical bulky substituent
(t-Bu) suffered from low reactivity (entry 8).
To explore the effect of bulkiness of the acyl group on BINOL,
we examined a series of 2-acyloxy-2′-vinyloxy-1,1′-binaphthyls
with Pd(OAc)2-4h as catalyst (Table 2). The nonacylated com-
pound 1b showed almost no selectivity (krel ) 1.1), although the
reactivity was extremely high (entry 1). The krel value was increased
by larger substituent as follows: acetyl (1c, krel ) 6.1, entry 2),
1-heptanoyl (1d, krel ) 14.3, entry 3), pivaloyl (1a, krel ) 20.3,
entry 4), and 1-adamantanoyl (1e, krel ) 28.7, entry 5).
Then, we investigated the reaction with various kinds of racemic
2-pivaloyloxy-2′-vinyloxy-1,1′-binaphthyls and 1,1′-biphenyls using
Pd(OAc)2-4h as catalyst (Table 3). The reaction system was
applicable to all examined substrates 1a and 1f-m, giving moderate
to high selectivity (krel ) 12.1-35.8). For example, the kinetic
resolution of 1i proceeded with krel values of about 30. The
9
10474
J. AM. CHEM. SOC. 2005, 127, 10474-10475
10.1021/ja051750h CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Kinetic Resolution of
the deference in reactivity and selectivity between ligand 4h and
4a was again observed (4a, krel ) 25.3, entry 8, cf. entry 5). The
highest krel value of 35.8 was observed with 1l (entry 11).
Although methanol was the best reagent in the present alcoholysis
reaction in terms of reactivity, the krel value was improved to 41.2
(1a) and 39.8 (1i) by the use of 2-chloroethanol as reagent (eq 1).
2-Acyloxy-2′-vinyloxy-1,1′-binaphthyl 1 Catalyzed by Pd(OAc)2-4h
Complexa
t
conv
(%)b
ee of 1
(%)c
ee of 2
(%)c
d
entry
BINOL
R
(h)
krel
1
2
3
4
5
1b
1c
1d
1a
1e
H
3
24
63
61
43
49
44
58
56
4
54
61
96
96
6
57
77
69
77
1.1
6.1
14.3
20.3
28.7
COCH3
CO(n-C6H13)
CO(t-Bu)
The reaction appeared to take place in a similar manner with
Pd-catalyzed transfer vinylation from vinyl ethers to alcohols,9
although our work is the first example of its asymmetric version.
Actually, 1-dodecyl vinyl ether was isolated using 1-dodecanol as
reagent, though the krel value was decreased to 3.6. We expect that
the reaction is irreversible. In fact, the vinylation of 2a by ethyl
vinyl ether using Pd(OAc)2-4h did not proceed.9
CO(1-adamantyl) 42
a The reaction was carried out with 1 M solution of 1, 5 mol % of
Pd(OAc)2, 10 mol % of (R,R)-4h, and 10 equiv of MeOH in CH2Cl2 at 20
°C. b Calculated from isolated yields of 1 and 2. c Determined by HPLC.
d From ref 10.
Table 3. Kinetic Resolution of Binaphthyls and Biphenyls
Catalyzed by Pd(OAc)2-4h Complexa
In conclusion, we have achieved palladium-catalyzed kinetic
resolution of various kinds of 2,2′-dihydroxy-1,1′-biaryls by alco-
holysis reaction of their vinyl ethers. The reaction was applicable
to 1,1′-bi-2-phenols as well as 1,1′-bi-2-naphthols with high
selectivity.
Acknowledgment. This work was supported by Grant-in-Aid
for Scientific Research on Priority Areas (No. 16033204, Reaction
Control of Dynamic Complexes) from Ministry of Education,
Culture, Sports, Science and Technology, Japan.
t
conv
(%)b
ee of 1
ee of 2
d
entry
biaryls
(h)
(%)c
(%)c
krel
1
2
1a
1f
1g
1h
1i
1i
1i
1i
1j
61
52
96
85
20
40
68
81
63
62
40
72
58
60
45
37
51
55
44
57
51
56
50
40
96 (R)
97 (R)
63 (R)
45 (R)
88 (R)
95 (R)
70 (R)
97 (R)
84 (+)
97 (+)
86 (R)
51 (R)
69 (S)
65 (S)
77 (S)
79 (S)
83 (S)
79 (S)
88 (S)
73 (S)
81 (-)
75 (-)
86 (S)
81 (S)
20.3
18.5
14.9
13.1
30.1
31.4
32.7
25.3
24.3
27.0
35.8
12.1
Supporting Information Available: Experimental procedures and
full characterization of new compounds (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
3e
4
5
6
References
7f
8g
9
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a The reaction was carried out with 1 M solution of 1, 5 mol % of
Pd(OAc)2, 10 mol % of (R,R)-4h, and 10 equiv of MeOH in CH2Cl2 at 20
°C. b Calculated from isolated yields of 1 and 2. c Determined by HPLC.
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mol % of (R,R)-4h used. g (R,R)-4a was used as ligand.
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J. AM. CHEM. SOC. VOL. 127, NO. 30, 2005 10475